A day for the history books? Quite possibly. Ben Bernanke on Wednesday night did what central bankers rarely do: he signalled his intentions clearly and said, in effect, that a turning point has been reached. The era of quantitative easing is about to draw a close in the US. If the economic data in the US continue to meet the Federal Reserve's expectations, the QE game will be over by the middle of next year.

Of course, Bernanke also littered his remarks with get-out clauses. "Our policy is in no way predetermined and will depend on the incoming data and the outlook," he said. But markets never waste time on niceties. If the Fed has a preferred plotline, investors react.

Thus the yield of 10-year Treasury bonds hit 2.4% – that's extraordinary when you consider that the level on 1 May was 1.63%. In the dry world of fixed-income investing, an earthquake has happened. Tremors elsewhere were inevitable. Share prices fell around the world, with the FTSE 100 index losing 3%. Prices of commodities, including gold, slumped and emerging market assets took a pounding.

Hold on, you might say, wasn't Bernanke announcing good news on the economy? Well, yes, he was. There has been "further improvement" in employment and the "downside risks" to the economy have diminished. That is why he thinks QE can be exited safely.

Investors are not convinced. First, they worry that the US recovery is not secure. As Gavin Friend, strategist at National Australia Bank notes, the data blows hot and cold. Estimates for second-quarter GDP in the US range from an annualised 1% to 2%, which is a very wide range.

Second, the Fed seemed to accompany its plan to exit QE with a weary shrug that this form of monetary medicine had reached its safe limits anyway. There was a danger that capital and assets would be mispriced, thereby creating bubbles and fresh dangers. But if the result is a stampede away from "hot," or QE-inflated, assets, there could trouble in store. Nobody really knows what happens when the adrenalin is withdrawn.

Third, the Fed wants to exit QE but central banks elsewhere, by and large, are still travelling the opposite direction. Could tighter US monetary policy upset others' plans? In the past, sharp rises in US bond yields have caused chaos in emerging markets. And this time there may also be a knock-on effect in the eurozone. Spain's 10-year yields are suddenly uncomfortably close to 5% again. Uncertainty is back.